The Gunning Fog Index gives the number of years of education that your reader hypothetically needs to understand the paragraph or text. The Gunning Fog Index formula implies that short sentences written in plain English achieve a better score than long sentences written in complicated language.

For reference, the New York Times has an average Fog Index of 11-12, Time magazine about 11. Typically, technical documentation has a Fog Index between 10 and 15, and professional prose almost never exceeds 18.